1. Field of the Invention
The invention relates to the field of plural-hull craft, and more particularly concerns an arrangement of independent hulls and interconnecting structure.
2. Description of the Prior Art
Plural-hull sailing craft, including outrigger canoes, proas, catamarans and trimarans, are often preferred over comparable monohull sailing craft because of their greater speed and lateral stability, and their shallower draft. The greater speed derives largely from the greater lateral stability which allows more sail to be carried than can be carried by comparable monohull craft.
Lateral stability, that is, stability against heeling or capsizing under lateral sail forces, is normally derived on monohull sailboats by a combination of buoyancy change with anle of Keel, ballast and/or weighted keels. When ballast or a weighted keel is used, as both generally are in larger craft, additional hull buoyancy, in the form of a larger single hull, is required to support the additional weight. This larger hull causes an increase in water drag which reduces maximum speed.
Plural-hull sailing craft, however, achieve excellent lateral stability almost exclusively from lateral separation of the hulls. Befor lateral sail forces can heeel such craft over, upwind hulls must necessarily be lifted from their normal buoyant positions. Buoyancy of lifted hulls is reduced and the resultant increased hull weight, acting at the separation distance from downwind hulls, exerts a restoring force which counteracts further heeling. Additionally, the more deeply submerged leeward hull exerts a buoyant righting moment. These restoring forces are macimized at heeeling angles which completely lift or fly the upwind hull or hulls free of the water.
Also, because of the comparatively light weight of at least smaller plural-hull craft, crew weight is commensurate with the weight of the craft and additional lateral stability can be achieved by optimum crew placement.
As plural-hull sailing craft normally do not employ weighted keels or carry ballast, they require less hull buoyancy than comparable monohull sailing craft and can use slender, shallow draft hulls with lower water drag. Also, inasmuch as these type craft have good lateral stability, they can carry much larger sails than comparable monohull craft with less danger of lateral capsizing. Lower hull drag and greater sail area account for the relatively high speed of plural-hulled craft.
Another advantage of plural-hull craft becomes apparent when theoretical hull speed -- a theoretical upper speed limit of hulls in knots equal to 1.4 times the square root of hull length in feet -- is considered. At this theoretical speed water drag abruptly and significantly increases, and greatly increased driving force is required to drive the hull faster. This theoretical hull speed limit is generally the limiting speed for monohull sailboats (except when planing) because at this speed there is usualy insufficient sail power in reserve to overcome the increased hull drag. This is generally not true for plural-hull sailing craft as the generally more slender hulls have lower drag at their theoretical speed limits, which limits are reached while the relatively larger sails still have sufficient force to overcome the increased drag.
However, the very design characteristics which account for shallow draft, high speed and good lateral stability -- widely separated, slender, shallow draft hulls, large sail areas and relatively light overall weight -- cause plural-hull craft to be extremely difficult to maneuver, particularly in putting about, to be longitudinally unstable and to be subject to sideways drift.
Monohull sailing craft tend to pivot about a vertical axis at the center of lateral resistance in turning and consequently are easily turned. However, each of the several hulls of plural-hull craft has its own center of lateral resistance about which the craft cannot simultaneously pivot. A compromise is struck, and the craft is said to pivot about the bow of one hull and to drag the remaining hulls through the turn. The sideways dragging of hulls causes considerable resistance to turning, this resistance increasing with the number of hulls and with increased hull separation.
In putting about through the wind to change tack, all craft operating under sail power alone depend upon forward momentum to carry them through the turn. Because of their large resistance to turning and their comparatively light weight, and hence comparatively lower forward momentum, many plural-hull sailing craft are difficult or impossible to put about without auxiliary power. This is a serious disadvantage of this type of craft.
Another serious disadvantage of most popular plural-hull sailing craft is their susceptibility to capsizing diagonally forward under large longitudinal sail forces which drive the bows of leeward hulls under water. This is particularly the case when simultaneous large lateral sail forces put most or all of the craft's weight on leeward hulls by flying the windward hulls. This susceptibility is caused by the use of relatively lower displacement hulls and relatively large sails on this type of craft.
Forward capsizing is not of serious concern in modern monohull sailing craft, for most sea conditions, because lateral sail forces, usually several times greater than longitudinal sail forces, are normally the limiting factor. Thus, if the amount of sail carried is insufficient to cause monohull craft to capsize sideways, it is generally to small to cause them to capsize forward; although, it was not unheard of for tall masted clipper ships to be driven bow first under water in a gale by use of too much sail.
A third, less serious, defect of most plural-hull craft, is their tendency to drift sideways under lateral wind forces because of the relatively low resistance to drift of their shallow draft, keelless hulls.
Improving maneuverability of plural-hull sailing craft by shortening hulls, using extreme rocker (longitudinal upward curving of hulls), and spacing hulls closer together, decreasing sideways drift by adding keels or using asymmetrical hulls, and improving longitudinal stability by increasing hull buoyancy and reducing sail area causes corresponding reduction of advantageous lateral stability, speed and shallow draft. Thus, the prior art has attempted to effect improvements in alternative manners.
Quadra-hull craft have been designed (e.g., Dismukes, U.S. Pat. No. 3,316,873, and Hamilton, U.S. Pat. No. 3,265,026) which are equivalent to craft formed by cutting both hulls of a catamaran in two along a transverse plane and spacing the hull segments longitudinally apart. Although longitudinal stability tends to be improved, in much the same manner as lateral hull separation improves lateral stability, water drag is considerably increased because of wave interference between the in-line hulls. Also, the number of hulls which must be driven past their theoretical hull speed limit, if reasonably high speeds are to be achieved, is doubled, as is the increased hull drag. These disadvantages are continuously present, even when wind forces do not require increased longitudinal stability.
Other designs (e.g. Barkla, U.S. Pat. No. 2,804,038) have added horizontal planes to the bows of forward hulls to provide additional bow lift to prevent bow submergence. Some (e.g., Fletcher, U.S. Pat. No. 2,238,464) have hinged forward hulls about transverse axes so the hulls may conform to wave motion to help prevent swamping by oncoming waves. However, such hull designs increase water drag under all sailing conditions, regardless of the need for additional longitudinal stability.
Some designs (e.g., Perge, U.S. Pat. No. 2,944,505 and McIntyre, U.S. Pat. No. 2,106,432), when sailing with the wind or on broad reaches, use an inclined sail to create a lifting force to counter the forces tending to drive the bows under water. This, however, is accomplished at a sacrifice of forward driving power.
Many plural-hull craft, often catamarans, outrigger canoes or proas, are designed with complete fore and aft symmetry (e.g., Smith, U.S. Pat. No. 3,295,487 and Laurent, U.S. Pat. No. 3,173,395) to circumvent maneuverability problems. Instead of putting about to assume a new tack, the sail is manually repositioned and the craft is "backed" into a new heading, the previous stern now acting as the bow and vice versa. Fore and aft symmetry is not, however, good hull or sail design, and performance is sacrificed under all sailing conditions, even when no maneuvering is required.
Some trimarans have been designed with horizontally rotatable hulls so the craft may be "skewed" onto a new heading rather than having to be put about (e.g., Henderson, U.S. Pat. No. 1,303,839 and Twining, U.S. Pat. No. 606,104). Individual hull rotation has sometimes been provided so that a hull or hulls may function as rudders (e.g., Creed, U.S. Pat. No. 1,422,542, Malrose, U.S. Pat. No. 3,112,725 and Lake, U.S. Pat. No. 1,846,602). Such craft are, however, not designed to improve longitudinal stability, and consequently sail area, and hence speed, cannot be maximized.
Resistance to sideways drift is often provided by centerboards (e.g., White, U.S. Pat. No. 3,223,064) or inclined planes (e.g., McIntyre, U.S. Pat. No. 1,356,300). However these, unless large, are relatively ineffective in reducing sideways drift and they increase draft. If they are large, weight and draft of the craft may be considerably increased.
Accordingly, it is an object of the present invention to obtain advantages of plural-hull craft while minimizing their disadvantages.